Everything works fine until, we don't know why, the ram preview is getting not realtime, like 30fps or under.

Sometimes purge memory do the trick but not always.

We need to reboot and it works again, but few minutes.

We made a test on CS5 and it works well realtime, but same problem on CS6.

I tried to delete all the preferences to check but no difference.

We are working with nfs network account but i always redirect disk cache to local disk (set to 20 gigas) and also media conformed cache (database and cache)

We are getting mad, it's really hard to work that way, rebooting each time.

Maybe an issue with Global performance cache introduced in CS6 ?

It's not a footage/project issue because the entire project (a blend of tiff, png, prores422) can be played 120 frames/sec without any problem on after CS5, after CS6 (when it works), CC (when it works).

The problem is when working on project, changing layers, adding effects, After CC starts not be slow and not realtime using Ram Preview.

Also, when we are previewing, a lot of ram is not used, it only uses like 20% of the ram.

60 fps is a very high frame rate not usually supported. If your footage is shot at 60i then it is probably 29.97 fps when converted to progressive video. If the footage is actually 60 fps then you will be better off, and by better off I mean more folks and streaming services will be able to view your video at the proper speed.

So my recommendation is to first determine your audience and then set up your comp appropriately. In all my years in the business I have only delivered one high frame rate project and that was 48 fps.

The other thing I would recommend is to optimize your system for AE. That does not necessarily mean throw all your available ram and processors at the app. There are some good recommendations here: Optimizing After Effects Performance